In the production of continuous webs of flexible materials, such as thermoplastic films, such films are conventionally wound on a cylindrical core until the desired length of material has been obtained. It has been a significant problem in the art to efficiently transfer the web material from a fully wound core to a fresh empty core for continued production.
It has been known in the art to provide means to transfer a continuous web from one windup core to another. Principally, these employ a plurality of cores or spindles which are mounted upon an indexable turret arrangement. Typically, when one core has been filled, the turret rotates the empty core into winding position, the web is stopped, cut from the full core and attached to the new core. The fresh core is then wound with web material.
A problem with this method is that a significant amount of production time is lost during the course of a day when the web must be stopped and started up again. Also, the constant attention and action of an operator is required during the course of this change-over. Furthermore, the windup operation is only the last step of a series of complex web production sequences. Typically, the prior production steps must be maintained at a continuous uninterrupted speed which cannot be stopped without serious production consequences. For example, biaxially oriented polyethylene terephthalate film is produced by continuously melting and extruding polymer onto a casting drum, then stretching and heating in a precisely timed sequence. Stopping, slowing or interrupting the production line therefore detrimentally affects many upstream operations with a consequential loss of production and valuable materials.
In an effort to avert these losses, various methods have been tried. One method is to store the continuously produced web material in an accumulator. Typically, these are a series of translatable rollers which spread apart and store the web produced during the core stoppage and then contract, giving up their stored web when the new core is in place and winding. This method is disadvantageous since the accumulator has only a limited storage capacity and itself must occupy a substantial amount of valuable production space. Also, overall production is still limited since web transfer must still take place with the web stopped or slowed at the windup station.
Subsequent methods have attempted to instantaneously cut and transfer the web to the new core in a single operation, thus essentially preserving a continuous production cycle. One such device is shown in U.S. Pat. No. 2,942,796. The problem with this instantaneous severing is, as is disclosed, that inertia must be overcome in starting up the new roller; that is, the new roller is stopped at transfer and then begins to pick up to its operating speed. During this time, upstream web production must still be accumulated by some appropriate method. Other methods employ pressing rollers and brushes in conjunction with travelling cutters to effect web transfer.